1. Field of the Invention
The invention relates to paving machines and, more particularly, relates to asphalt paving machines having heated screeds for leveling asphalt.
2. Discussion of the Related Art
The sole plates of screeds of asphalt paving machines are typically preheated to a temperature of about 200.degree. F. to 300.degree. F. before paving commences and are maintained at this temperature during paving to prevent the hot asphalt being leveled by the screed from congealing on the face of the sole plate screed. Screeds have traditionally been heated by oil or gas burners mounted above the screeds such that the flames from the burners impinge sheet metal plates on top of the screeds. Such burners supply intense heat to localized portions of the screeds which results in uneven heating and, if not carefully controlled, may warp and render ineffective the screeds.
Systems have been proposed which are designed to avoid or to at least alleviate the problems associated with traditional screed heaters One such system is disclosed in U.S. Pat. No. 5,7096,331 to Raymond. The heater proposed by Raymond heats the sole plate of the screed of a paving machine via heat transfer from heating oil stored in a low pressure reservoir mounted directly on top of the sole plate screed. Oil in the reservoir is drawn from the reservoir, pressurized by a high pressure pump, and then fed through a pressure release valve or other suitable flow restrictor which creates a pressure drop in the range of about 700 to 800 psi, thereby heating the oil to a temperature of about 275.degree. F. The thus heated oil is then returned to the reservoir for heat transfer to the sole plate of the screed.
The system proposed by Raymond suffers from several drawbacks and disadvantages. Lost notably, the large pressure drops needed to provide the necessary heating require that the heating oil be pressurized by a pump to a relatively high pressure in the range of 800 to 1000 psi before undergoing the pressure drop in the flow restrictor. This requires the use of high pressure hoses and connections throughout the system, thus increasing the cost and complexity of the system and also increasing the dangers of leaks which could render the system ineffective. Moreover, if for any reason the pump and relief valve are not capable of providing a sufficiently large pressure drop to adequately heat the oil, the system is incapable of boosting the oil temperature to the required level.